In semiconductor production, wafers are sequentially processed in a plurality of process steps during the production process. With increasing integration density, demands in terms of the quality of the features configured on the wafers are rising. It is advantageous, for this purpose, if the quality of even individual process steps, for example lithography steps, can be reliably assessed during the manufacturing process and before any subsequent process step. Thus, if a determination is made, just after a process step is performed and even before a production process has been completed, that a wafer or features configured on the wafer are defective, the wafer can be immediately discarded with no need to perform additional subsequent process steps. Or wafers found to be defective can be reprocessed separately until satisfactory quality is achieved. Efficiency and yield in semiconductor production can thereby be enhanced.
Optical apparatuses are particularly suitable for inspecting the surface of wafers. Optical apparatuses are known that, by image recognition, can recognize a wide variety of features on the surface of a wafer. The wafer is usually illuminated in bright-field fashion in this context, and scanned with a camera (matrix or linear camera). In one often-used type of wafer inspection apparatus according to the existing art, the surface of the wafer is illuminated stroboscopically. A region on the surface of the wafer is illuminated by a light flash, an image of the illuminated region is acquired, and the wafer and illuminating light beam are displaced relative to one another for a subsequent image acquisition operation.
Intensity fluctuations of the light flashes used for illumination can have a disruptive effect on image evaluation accuracy in this context. For example, threshold values can be defined for image evaluation, a defect on the surface of the wafer being indicated only if those values are exceeded. Intensity fluctuations in the vicinity of the threshold value thus degrade the image evaluation accuracy. Intensity fluctuations of the light flashes used for illumination are troublesome also because they suggest imprecise operation of the wafer inspection apparatus, for example when sequentially acquired images are compared with one another.
The inventors have observed that in ordinary flash light sources, for example xenon flash lamps, flash-to-flash intensity fluctuations of approximately 5% or more can occur.